The present invention relates to brake-boosters, especially for motor vehicles, and applies more particularly to adjusting the jump of such boosters.
A brake-booster, as defined above, generally comprises a control rod which is displaced towards the front of the vehicle when its driver actuates the brake pedal. This displacement of the control rod is transmitted to an element called a plunger which in turn actuates boosting means. In general, these boosting means consist of a three-way valve, the function of which is to interrupt communication between the front and rear chambers of the booster, in which the same reduced pressure prevails in the rest position, and to put the rear chamber in relationship with a source of pressure under a higher pressure, so as to generate a pressure difference on the two faces of a piston separating these two chambers. The piston is then displaced forwards, being fixed to an output rod or push rod which in turn acts on a master cylinder then generating a pressure increase in the hydraulic brake circuit of the vehicle, in order to brake the latter.
In a known way, the piston serving for transmitting the boost force to the output rod or push rod acts on the latter via a reaction disk produced from a deformable material, such as an elastomer. At rest, a slight play is present between the front end of the plunger and the reaction disk. The rear end of the plunger forms an annular valve seat coaxial with another annular valve seat formed on the piston, a shutter-forming member interacting with these valve seats.
In such boosters, the reaction at the brake pedal begins to appear only when the boost force generated by the booster exceeds a particular threshold. This threshold is called the "jump" of the booster. It is an important feature of the booster. In fact, although it is necessary for the jump to be present in order to ensure an immediate response of the brakes whenever the pedal is actuated, vehicle manufacturers generally want the value of this jump to remain within particular limits, so that the boost does not reach too high a value without an increase in the reaction of the brake pedal.
However, particularly in view of the production tolerances of the various component parts of the booster, there can be considerable differences in the value of the jump from one booster to another in series production.
The document GB-A-2 074 270 makes known a process for adjusting the value of the jump of a booster, according to which process the play present between the front face of the plunger and the reaction disk is adjusted. This process has many disadvantages, in that it is long and complicated and, moreover, to carry it out it requires the use of ultrasonic or heating means for subjecting to plastic deformation a valve part comprising an additional component between the plunger and the reaction disk. The major disadvantage of this process is, furthermore, that the adjustment has to be made before the reaction disk and consequently the booster itself is assembled. As a result of this, the various components have to be paired with great care.